1. Field of the Invention
The present invention relates to an imaging device which creates half-tone reproductions of an original image using an area gradation method by assigning multiple dots to each single pixel.
2. Description of the Prior Art
In digitally controlled CRT displays, printers, and copying machines, images are formed by serial dot arrays (output pixels) comprising displayed and non-displayed dots.
Two methods are commonly used in this type of device to reproduce gradations in the tonal density of the original image, specifically density gradation in which the size and toner density of the dots are changed, and area gradation in which the number of dots within a single unit area is changed according to the density of the original image. Of these two methods, area gradation methods, typically density pattern processing and dithering, are the most commonly used due to their relative ease of control.
In density pattern processing, each pixel (input pixel), i.e., the smallest readable unit of the original image during the imaging process, consists of a dot matrix comprised of multiple dots, and the density pattern best suited to accurately reproducing the density of the original is selected from among a number of predetermined density patterns, thereby determining the specific series of dots in the dot matrix to be used. While this makes it possible to produce optimized half-tone images, this method also requires a large capacity memory area to store all possible density patterns.
In dithering, one dot is assigned to each single pixel, and each pixel is digitized based on a variable level threshold value. This method is suited to applications in which the pixel size is small, as in high resolution imaging processes based on a digital signal produced from high resolution scanning of the original image.
Systematic dithering, in which the dither data group (dither matrix) of regularly changing values is repeated, is used most frequently. With this method, only one dither matrix needs to be provided, and large capacity memory devices and complex processing circuits are not required.
However, because the dither matrix area representing a single half-tone unit area in the processed image represents multiple pixels, the resolution of the image reproduced according to the resolution used when scanning the original image is eventually low, and as the dither matrix area is increased to increase the number of halftones, the overall image resolution decreases further.
As a result, conventional imaging devices use an area gradation method combining dithering with density pattern processing. In this process, when a dither matrix area is assigned to multiple pixels, the number of halftones can be increased without increasing the dither matrix area by assigning multiple dots to each single pixel in the dither matrix for a single unit area of the original image.
For example, in the device described in U.S. Pat. No. 4,814,886, a series of (m) dots long is assigned to each single pixel; a dither matrix comprised of (a.times.a.times.m) threshold data values is assigned to each (a.times.a) pixel area with (a) pixels in the main scanning direction and (a) pixels in the sub-scanning direction, thus forming half-tone reproductions with (a.times.a.times.m+1) half-tones (a case when the number of display dots is 0 is counted as one tone) in the tone unit area.
In a conventional imaging device, the dither matrix area, i.e., the half-tone unit area, corresponds to a specific integer number of pixels. Thus, when the pixel density varies in the lateral and longitudinal directions in order to fractionalize the original image, the half-tone unit areas in the lateral and longitudinal directions of the reproduced image are not of equal density, and a large difference in the resolution in the lateral and longitudinal directions results.
Therefore, in a digital camera, for example, differences in the image quality of the photographed image will occur even with the same original due to the direction of the original table.
In addition, in a conventional imaging device, display dots are added one at a time to one side of a single display dot referred to as the nucleus of growth through the imaging area (which is equal to (m.times.a) dots in the dither matrix area, i.e., the area of one horizontal matrix) as the density of the original image increases. As a result, the dot which is the minimum threshold data (i.e., the growth nucleus) and the dot which is the maximum threshold data are adjacent within the (m.times.a) threshold data group corresponding to the area covered by the dither matrix. Thus, when images are formed by repeating the dither matrix, a specific edge pattern is regularly formed by the above adjacent dots, and problems result in the reproducibility of the original image.